All persons born in the U.S. are automatically granted U.S. citizenship. People who are not U.S. citizens by birth may obtain U.S. citizenship through the process of naturalization, which requires the fulfillment of a series of criteria codified in the Immigration and Nationality Act (INA). The process can take anywhere from six months to two years. A legal immigrant who wishes to naturalize must be over 18 years old, must have lived in the U.S. for at least five years (three years if married to a U.S. citizen) and have no criminal record. Additionally, candidates must demonstrate English language proficiency and knowledge of U.S. history and government by passing a naturalization test.
Historically, less than half of all immigrants to the U.S. have become citizens. While the total foreign-born population has increased in the course of the last four decades, the proportion of naturalized foreign born has declined from 63.6% in 1970 to 43% in 2008. In general, persons who arrived in the United States in earlier decades are more likely to naturalize or to have naturalized than those arriving more recently. Furthermore, the proportion of naturalizations is higher among individuals who possess a bachelor's degree than among those who lack a high school diploma.
In 2011, a total of 694,193 persons obtained U.S. citizenship; the top five countries of birth of these new citizens were Mexico (94,783), India (45,985), the Philippines (42,520), China (32,864) and Colombia (22,694). The largest number of people who were naturalized in 2011 lived in the states of California (151,183), Florida (87,309), and New York (76,603).